In recent years, ink-jet recording methods have found wide application in the field of various kinds of graphic arts such as photography, various kinds of printing, marking and specific printing such as color filters due to its ability to form images simply and cheaply. Particularly, it has also become possible to obtain image quality comparable with silver salt photography by utilizing a recording apparatus which ejects and controls minute dots; ink in which color reproduction range, durability and ejection suitability have been improved; and exclusive paper in which ink absorption, color forming property of the colorant and surface gloss have been greatly enhanced. Image quality improvement of current ink-jet recording methods has been achieved only when complete set of a recording apparatus, ink and exclusive paper are employed as a system.
However, an ink-jet system which requires exclusive paper is problematic with respect to limitations of recording media and increased cost of such recording media. Therefore, many attempts have been made to record on a medium, on which ink is to be transferred, differing from exclusive paper by means of ink-jet recording. Specifically, there are phase-conversion ink-jet methods utilizing wax which is solid at room temperature, solvent-type ink-jet methods utilizing an ink which is comprised mainly of a rapid-drying organic solvent, and UV ink-jet methods in which an ink is cross-linked by ultraviolet (UV) light after recording.
Among them, a UV ink-jet method has been noted recently in respect to relatively low odor compared to a solvent-type ink-jet method, rapid drying property and capability of recording on a recording medium without ink absorption property; UV-curable ink-jet ink is disclosed (e.g., Patent Document No. 1.)
However, even when these inks are used, because a dot diameter after an ink impact is largely changed due to the kind of the recording material or the working environment, it is impossible to form the highly minute image onto all the recording materials.
As ultraviolet ray hardenable ink, it is known a radical polymerization type and a cationic polymerizable type.
The cationic polymerizable type is focused because of the following reasons: polymerization reaction is not affected by the presence of an oxygen atom; among employed cationic polymerizable monomers, ionic polymerizable monomers such as oxetane and epoxy compounds emit low amount of smell and show small amount of shrinkage.
The cationically polymerizable type UV ink tends to be affected by humidity since it depends on ionic polymerization utilizing acid generating agents as a catalyst. Further, it results in a problem in which temperature dependency is high due to the fact that activation energy of the reaction is greater than radical reactions.
Commonly, other than the above-mentioned ultraviolet radiation curable type ink-jet ink, cationically polymerizable compositions are widely employed in resist materials, coating materials, and printing inks such as flexographic ink.
Further, employed as acid catalysts are thermally generating type and photo-generating type acid generating agents. Listed as representative compounds are phosphonium salts, sulfonium salts, iodonium salts, and sulfonic acid esters.
Generally, in a cationic polymerization reaction, acid acts as a catalyst. Therefore, when acid is once provided into the system from polymerization initiators, polymerization is induced over an extended period of time due to lack of deactivation, whereby storage stability is very adversely affected. Consequently, the storage stability of the above-mentioned cationically polymerizable compositions becomes a critical problem.
As a method to improve the storage stability of cationically polymerizable compositions, a very effective common method is one in which a small amount of base is previously incorporated as a polymerization inhibitor. However, the following problems have occasionally occurred. The polymerization inhibitors decrease speed. No effects result depending on selection of polymerizable monomers and initiators. Specifically, it is difficult for thermally acid generating type phosphonium salts or photolytically acid generating type iodonium salts to retard thermal acid generation from the initiators. As a result, improvement of the storage stability has been limited. Further, when oxetane compounds are employed as a cationically polymerizable compound monomer, a termination reaction rarely occurs. As a result, problems have occurred in which the viscosity of the cationically polymerizable compositions increases to an extreme due to excessive polymerization.
Further, a photopolymerizable composition in which triarylsulfonium salts are employed as an initiator is the most stable system and is also very useful for ink-jet ink which requires low viscosity as well as high stability. However, even though the triarylsulfonium salts are employed, a high speed polymerizable composition in which oxetane compounds and epoxy compounds are simultaneously used has not resulted in desired storage stability over an extended period.
As noted above, from the aspect of the composition which constitutes cationically polymerizable compositions, trials have been attempted to improve stability, for example, by selecting the type of polymerization initiators. However, desired stability has not been achieved under various environmental conditions.
As a means to improve storage stability of polymerizable compounds, trials have been performed in which by selecting materials of storage containers or the characteristics, effects due to temperature, ambient light, heat, and air during storage are decreased. For example, proposed are (refer, for example, to Patent Documents 1 and 2) a method in which a photosensitive colored resinous composition comprising colorants, binder polymers, photopolymerizable compounds, photopolymerization initiators, and solvents is stored at −10-30° C. upon being vibrated, a method in which, in a storage container, the product of the ratio (being a void ratio) in percent of the void volume to the interior volume of the container and the partial oxygen pressure (hPa) in the gas in the void exceeds the specified conditions, or a method in which storage is carried out in a container which is below the specified light transmittance conditions in the wavelength region of 250-440 nm. However, as a storage method for the cationically polymerizable composition in which cationically polymerizable monomers are employed as a polymerizable monomer and acid generating agents are used as a polymerization initiator, none of these methods have resulted in the desired quality.
In order to overcome the above-mentioned drawbacks, various improvement means have been investigated. For example, disclosed is an actinic radiation curable type ink-jet ink which incorporates cationically polymerizable compounds such as oxetane ring containing compounds (refer, for example, to Patent Documents 3 and 4). However, even in each of the proposed inks, the above-mentioned drawbacks are not sufficiently overcome due to large effects of water incorporated into the ink.
On the other hand, a method for improving the storage stability of ink by the addition of basic compounds is disclosed (refer, for example, to Patent Documents 5 and 6). The inventors of the present invention conducted investigations based on the methods disclosed in Patent Documents 5 and 6. As a result, it was discovered that the storage stability resulted in improvement effects, though the effects were not sufficient, and the curability of the ink was degraded due to the addition of the basic compopunds.
(Patent Document 1)
Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 2003-107727 (claims)
(Patent Document 2) JP-A No. 2003-149829 (claims)
(Patent Document 3) JP-A No. 2001-220526 (claims)
(Patent Document 4) JP-A No. 2002-188025 (claims)
(Patent Document 5) JP-A No. 2000-186079 (claims)
(Patent Document 6) JP-A No. 2000-327672 (claims)